This invention relates generally to Auger spectrometry, and more particularly to an electronic system for measuring the number of electrons emitted from a surface under investigation from a signal output of an electron detector.
Auger spectroscopy is a technique for determining the composition of a material surface by bombarding that surface with an electron beam and then detecting the electrons emitted from it. A spectra of a quantity of electrons emitted as a function of their energy can tell the characteristics of the material. These Auger electron lie in an energy level that extends gernerally between the low energy level of the emission of secondary electrons up to the energy of the impinging electron beam. In this region, small peaks will occur in the spectra at certain energy levels that identifies the existence of certain elements in the surface. A particular application of Auger spectroscopy is to examine the depths of a material upon peeling layers away and examining the new surface.
Available detectors, such as a cylindrical analyzer, coupled with an electron multiplier, provides a voltage signal at its collector that is proportional to the number of electrons that have entered the detector with an energy level equal to the voltage to which the detector is set. Because high voltages are necessary to operate the detector and multiplier, the voltage variation at the output of the detector that is desired to be measured, within a small fraction of one volt, are superimposed on hundreds or even thousands of volts. The problem in measuring a spectra under these circumstances is that the small voltage variations, which give an indication of the materials present, are lost in system noise, drift, etc.
One approach that has been taken in the past to extract the desired small peaks from the total amount of output signal is to measure the voltage drop across a load resistor at an output of the electron multiplier. Since this signal is superimposed on the several hundred or thousands of volts of the electron multiplier power supply, an isolation amplifier, chopper and analog optical coupler are used so that the desired voltage signal can be processed with respect to ground potential according to normal techniques. But the accuracy of these techniques is still not satisfactory because of drift and linearity inaccuracies of these coupling components. The measurement also takes a considerable time to accomplish by digital counting techniques.
Another approach that is utilized to overcome the problem is to differentiate the signal output of the electron multiplier in order to detect the location of the small voltage peaks in the baseline voltage curve. But this technique also suffers from an inability to detect very small signals and further does not enable the observer to determine the area of the peaks above the baseline, a desired quantity for certain analytical work.
Yet another technique for determining where the peaks of interest lie is to substract from the spectra signal a signal representing the baseline. However, this also has difficulties in detecting the peaks because of the high level of noise with respect to the level of the signal to be detected.
Therefore, it is a primary object of the present invention to provide a technique of processing an output signal of an Auger spectrometer detector which is fast, accurate and simple.